1. Field of the Invention
This invention relates to radar systems, and more particularly to an apparatus and method for rejecting rain clutter in a radar system used to detect the presence of obstacles in regions that are difficult to see by a host vehicle operator.
2. Description of Related Art
A problem that continues to plague automotive vehicle operators is difficulty in observing obstacles or other vehicles proximate the operators' vehicle but in locations or regions that are difficult to observe from the operators' seat within the vehicle. Such locations or regions that are near a vehicle yet not directly observable from the driver's seat are commonly referred to as "blind spots." For example, the region between the angles of 90.degree. and 170.degree., measured with respect to the forward direction of a vehicle in a clockwise direction (i.e., generally to the right of the vehicle and slightly behind the operator's seat), is typically a blind spot, particularly for large vehicles such as buses and trucks. Failure of an operator to be aware of an object (typically another vehicle) in this right-side blind spot when making a right-hand turn or a right lane change is a source of numerous accidents. Another common blind spot is the region directly to the rear of a vehicle. This region is of particular interest when the vehicle is in reverse gear (i.e., when "backing up"). Therefore, it is critical for the safe operation of a motor vehicle that the operator of the vehicle is able to detect obstacles (especially other vehicles) that are located in the operator's blind spots.
One prior art attempt at solving the problem of blind spot obstacle detection uses mirrors to aid the vehicle operator in detecting the presence of obstacles that might pose a hazard. Such mirrors have been made in a variety of shapes and having a variety of lenses. In addition such mirrors have been mounted in various locations to provide the operator with the greatest ability to detect the presence of obstacles in particular blind spots. For example, concave mirrors are commonly mounted to the right side of a vehicle and aimed at the right-side blind spot.
Mirrors provide the operator with some information regarding the presence of obstacles in certain of a vehicle's blind spots. However, disadvantageously, mirrors are less useful at night and under adverse weather conditions. Even under the best of conditions, mirrors that distort the reflection are typically required to allow the operator to view the right rear blind spot. Some operators find it difficult to properly interpret the image that is presented in such mirrors (such as convex mirrors that are commonly used as right side mirrors). Further, mirrors tend to reflect the headlights of vehicles approaching from behind and thus blind the driver of the vehicle on which the mirror is fixed. Hence, a more complete and satisfactory solution is desired.
One known alternative to the use of mirrors to detect obstacles in a vehicle's blind spot is to mount a camera on the vehicle to provide the operator with a visual image of obstacles in the vehicle's blind spot. However, this solution is complex and expensive, requiring a video camera and video monitor. Further, a video monitor can present a complex image, which, even if undistorted, can be difficult to interpret rapidly under stressful conditions that occur during heavy traffic conditions. Still further, monitors can be distracting. Moreover, like mirrors, such camera systems are less useful at night and under adverse weather conditions such as rain, sleet, or snow.
Another alternative to the use of mirrors is to direct radar transmissions toward each blind spot. Reflections of the radar transmissions can then be detected to determine the presence of obstacles in each of the blind spots. One such system is disclosed in U.S. Pat. No. 5,325,096, issued on Jun. 28, 1994 to Alan Packett and assigned to the owner of the present invention, which is hereby incorporated by reference. These systems use a common radar transceiver that transmits a radio frequency (RF) signal into a vehicle's blind spot. The transmitted signal is reflected by obstacles that are present in that blind spot region. The frequency of the transmitted signal is compared with the frequency of a reflection of the transmitted signal which is received within the radar system to determine whether the reflected signal has been Doppler shifted. A Doppler shift in the frequency generally indicates that an obstacle is present in the blind spot.
Disadvantageously, such Doppler radar blind spot sensors frequently generate false warnings (i.e., detect false targets) when used in adverse weather conditions, especially when used in the rain. There are two principle sources for the false alarms: (1) rain clutter produced by rain falling within a close range of the radar sensor; and (2) reflections from wet road surfaces, wet "non-road" surfaces, and wet foliage on the sides of the roads. Disadvantageously, the prior art vehicular radar systems misinterpret rain clutter, wet road surfaces and wet foliage as dangerous target objects. Consequently, the prior art radar systems falsely warn the driver of the existence of an object in the driver's blind spot. This creates a nuisance condition for the driver. The rain clutter, wet road conditions and wet foliage that the host vehicle (i.e., the vehicle equipped with the radar system) passes causes the radar system to falsely indicate the presence of an object in the host vehicle's blind spot even when no real threat exists. This can cause the operator of the host vehicle to lose faith in the reliability of the radar system and leave the system ineffective for warning the operator of real threats. In addition, such indications are distracting and disturbing to the operator.
Accordingly, a need exists for a simple, inexpensive solution to the problem of detecting hazardous obstacles in the blind spots of a vehicle. Such a solution should also be useful at night and under adverse weather conditions and should not generate nuisance conditions in response to rain clutter, wet road surfaces and wet foliage on the sides of the road as the host vehicle passes. The present invention provides such a solution.